this does the filtering in RoomList instead of RoomSubList, so
we can hide sections that don't have any results.

The filtering does happen with every rerender of RoomList,
but only does something while searching, so the performance
implications are probably negligible.

also remove collapse/expand button in search field

text inputs are now styled at every occurence in the app,
style input by default, and provide a .mx_textinput class
if buttons need to appear inside the input styling.

Before this was partially done in _common.scss, but as it's
highly theme-dependent, makes more sense to do it in the theme.

as we can reuse it in the room header